For receivers in radio communication systems, it is important to control the dynamic range of the signals at each processing stage in the receiver. High performance receiver designs utilize Automatic Gain Control (AGC) to tune the gain distribution, so the total received signal does not saturate at any stage in the receiver chain. Examples of devices that impact on the dynamic range are nonlinear amplifiers, limiting amplifiers, and Analogue-to-Digital (A/D)-Digital-to-Analogue (D/A) converters etc. FIG. 1 illustrates an example of a nonlinear amplifier. The characteristics seen in the figure are caused by noise floor and gain compression. FIG. 2 illustrates an example of a limiting amplifier where the characteristics seen are caused by limited output. FIG. 3 illustrates the dynamic range limited device characteristic of an A/D-D/A Converter.
FIG. 4 is a block diagram illustrating an example of an AGC receiver architecture. An input signal x(n) is received via the antenna 401, which goes through a receiver antenna filter h0(n) 402. An attenuator 403 adjusts the input signal and waits for a discontinuity to propagate. The adjusted signal proceeds through a band pass filter h1(n) 404, and then through a dynamic range limited device 405. Examples of dynamic range limited devices 405 are described above in relation to FIGS. 1, 2, and 3 above. The signal is processed through an anti alias filter h2(n) 406, before it is treated in a wideband power meter 407, where the wideband signal level is measured. The measured signal level is controlled by a controller 408 to determine if the signal is acceptable with regards to a threshold and/or range. If the signal is not acceptable, it is transmitted back to the attenuator 403. If the signal is acceptable, it is transmitted to a compensator 409, where the controlled signal together with the signal from the anti alias filter h2(n) 406 is processed. The compensator 409 changes the signal gain based on the controlled range. The compensated signal is transmitted through the channel filter h3(n), and the output signal y(n) is processed in a demodulator 411 to extract the original information bearing signal from the processed signal y(n).
Tuning or switching the gain at any processing stage, during the receipt of a continuous signal, may add discontinuity to the received signal. When the discontinuous signal is processed in the subsequent stages, depending on the processing algorithms of these stages, the discontinuity may add noise to the signal and also widen the spectrum of the received signal. The problem is increasing when a small wanted signal is received at the same time as a large interferer is received at a frequency which is close to the wanted signal frequency. In this case, the spectrum widening of the large unwanted signal may add a significant amount of noise and interference to the wanted signal. Such a noise pulse and distortion may cause estimation error in the demodulation process and ruin the interpreted symbol.